Metal contacts to group IV semiconductors by inserting interfacial atomic monolayers

ABSTRACT

Techniques for reducing the specific contact resistance of metal—semiconductor (group IV) junctions by interposing a monolayer of group V or group III atoms at the interface between the metal and the semiconductor, or interposing a bi-layer made of one monolayer of each, or interposing multiple such bi-layers. The resulting low specific resistance metal—group IV semiconductor junctions find application as a low resistance electrode in semiconductor devices including electronic devices (e.g., transistors, diodes, etc.) and optoelectronic devices (e.g., lasers, solar cells, photodetectors, etc.) and/or as a metal source and/or drain region (or a portion thereof) in a field effect transistor (FET). The monolayers of group III and group V atoms are predominantly ordered layers of atoms formed on the surface of the group IV semiconductor and chemically bonded to the surface atoms of the group IV semiconductor.

RELATED APPLICATIONS

The present application is a Continuation of U.S. patent applicationSer. No. 15/684,707, filed 23 Aug. 2017, which is a Continuation of U.S.patent application Ser. No. 15/146,562, filed 4 May 2016, now U.S. Pat.No. 9,755,038, which is a Continuation of U.S. patent application Ser.No. 14/360,473, filed 23 May 2014, now U.S. Pat. No. 9,362,376, which isa National Stage under 35 U.S.C. 371 of and claims priority toInternational Application No. PCT/US2012/060893, filed 18 Oct. 2012,which claims priority to and incorporates by reference U.S. ProvisionalApplication No. 61/563,478, filed 23 Nov. 2011.

FIELD OF THE INVENTION

The present invention relates to techniques for reducing the specificcontact resistance of metal—semiconductor (e.g., group IV semiconductor)junctions by interposing a monolayer of group V or group III atoms atthe interface between the metal and the semiconductor, or interposing abi-layer made of one monolayer of each of group V and group III atoms,or interposing multiple such bi-layers.

BACKGROUND

As the size of transistors is reduced to nanometer scale dimensions, forexample in the form of ultra-thin body (UTB) silicon-on-insulator (SOI)field effect transistors (FETs), FinFETs and nanowire FETs, the unwantedresistance associated with transistor sources and drains becomes an everincreasing burden on the performance of these devices and of theintegrated circuit products manufactured using such transistors.Furthermore, a reduction of dopant activation is predicted theoreticallyand demonstrated experimentally when the transistor source and drainregions are reduced in size below approximately 10 nm. By dopantactivation, we mean desired free carrier (electron or hole)contributions from deliberately introduced impurity species in asemiconductor host. This decrease in nanoscale dopant activation furthercontributes to undesirably high resistance of doped source/drain (S/D)regions both at the nanoscale metal contacts and within the bulk portionof the nanoscale doped regions. The resistance of metal contacts to asemiconductor increases if effective doping in the semiconductordecreases, the increase being primarily due to the presence of aSchottky barrier at metal-semiconductor contacts.

It is known that a high concentration of doping in a shallow region ofsemiconductor proximate to a metal-semiconductor interface can reducethe resistance of the metal-semiconductor contact by decreasing thewidth of the Schottky barrier. Although it is the barrier width that isreduced, from an electrical response point of view (for examplecurrent-voltage measurement), it appears that the Schottky barrierheight is reduced. An early article that describes this “effectivebarrier height” reduction by surface doping is by J. M. Shannon,“Control of Schottky barrier height using highly doped surface layers”in Solid-State Electronics, Vol. 19, pp. 537-543 (1976). It is alsoknown that a high concentration of dopant atoms can be introduced into ashallow region of a semiconductor proximate to a metal contact byso-called dopant segregation out of a metal silicide. A. Kikuchi and S.Sugaki reported in J. Appl. Phys., Vol. 53, No. 5, (May 1982) thatimplanted phosphorus atoms piled up near a PtSi-Si interface during PtSiformation and reduced the measured height of the Schottky barrier ton-type silicon. The reduction of the measured (effective) barrier heightof the Schottky diode was attributed to piled up phosphorus atoms in thesilicon causing the barrier to be more abrupt. That is, the result wasattributed to the effect described by Shannon in 1976.

For the past several decades the silicon microelectronics industry hasrelied on high doping concentrations in the silicon proximatemetal-silicon contacts as a means of obtaining acceptably low contactresistances to transistor sources and drains. The contact metal has forthe most part been a metal silicide, most recently nickel silicide ornickel platinum silicide. This approach to minimizing contact resistanceis expected to be insufficient in the future as the transistordimensions continue to shrink and the contact resistance becomes alarger portion of the total resistance between the source and drain(hence becoming a serious performance-limiting factor). The most recentInternational Technology Roadmap for Semiconductors (ITRS), published in2011, reports that there is no known solution to the contact resistanceproblem in bulk MOS transistors when the transistor gate length scalesto 18 nm, as expected in year 2014, and a specific contact resistance ofno more than 1.0×10⁻⁸ Ohm·cm² is specified. It is becoming increasinglyapparent that the Schottky barrier at metal-semiconductor contacts mustbe reduced in order to reduce the contact resistance to acceptablelevels, i.e. well below 1.0×10⁻⁸ Ohm·cm² in the case of MOS transistordoped source/drain contacts. A technology that is capable of reducingthe Schottky barrier and hence reducing the resistance of contacts todoped semiconductor regions may also be applied to so-called “metalsource/drain transistors” which do not have doped source and drain butrather utilize a direct contact between the metal and the transistorchannel (the region of free carriers that are modulated by theelectrical potential on a gate and that transport current between thesource and the drain).

A body of work published in 1991-1992 reported experimental verificationof theoretical predictions by Baroni, Resta, Baldereschi and others thata double intralayer formed by two different elements would create aninterface dipole, capable not only of modifying heterojunction banddiscontinuities, but also of generating band discontinuities inhomojunctions. McKinley et al. first reported obtaining 035-0.45 eV bandoffsets at {111}-oriented Ge homojunctions using Ga—As dipoleintralayers in a 1991 article “Control of Ge homojunction band offsetsvia ultrathin Ga—As dipole layers”, J. Vac. Sci. Technol. A 9 (3),May/June 1991 and in a similar article in 1992 “Control of Gehomojunction band offsets via ultrathin Ga—As dipole layers”, AppliedSurface Science Vol. 56-58, pp. 762-765 (1992).

Arsenic, gallium, and germanium depositions were done at roomtemperature on p-type Ge(111) substrates. Valence band offsets weremeasured by in situ core level x-ray photoluminescence. The deposited Geregion (overlayer) had a valence band offset to the Ge substrate asmanifested by a splitting of the Ge 3d core level into two components;one due to the Ge substrate and the other to the Ge overlayer. Bothpositive and negative valence band offsets were obtained in Gehomojunctions by introducing Ga—As dipole intralayers with either“Ga-first” or “As-first” growth sequences. The band offset was found tobe 0.35-0.45 eV with the Ge valence band edge on the As side of thejunction at a lower energy (i.e., more bound). Dipole intralayers wereexplained on the basis of the Harrison “theoretical alchemy” modeldescribed by W. A. Harrison et al. in “Polar Heterojunction Interfaces”,Phys. Rev. B 18, 4402 (1978). Intralayer control of band discontinuitieswas thus applied to homojunctions, expanding the potential domain ofband offset engineering beyond semiconductor heterojunctions.

In 1992, Marsi et al. followed up on the reports by McKinley et al. withthe articles “Microscopic manipulation of homojunction band lineups”, J.Appl. Phys., Vol. 71, No. 4, 15 Feb. 1992, “Homojunction banddiscontinuities induced by dipolar intralayers: Al—As in Ge”, J. Vac.Sci. Technol. A 10(4), July/August 1992 and “Local nature of artificialhomojunction band discontinuities”, J. Appl. Phys. 72 (4), 15 Aug. 1992.In the first article, Marsi et al. reported valence-band discontinuitiesat Si—Si and Ge—Ge homojunctions when III-V double intralayers of atomicthickness were inserted at the interfaces. Valence band discontinuitieswere again measured by in situ core level x-ray photoluminescence. In Gesamples, a deposited Ge region (overlayer) had a valence band offset toa Ge substrate as evidenced by a splitting of the Ge 3d core level intotwo components and a deposited Si region had a valence band offset to aSi substrate as evidenced by a splitting of the Si 2p core level. Theobserved discontinuities with magnitudes in the range 0.4 to 0.5 eV (forexample 0.5 eV for Si—P—Ga—Si and 0.4 eV for Si—P—Al—Si) were inqualitative agreement with theoretical predictions although mosttheories estimate larger valence band discontinuities due to the dipoleeffect. A III-V intralayer at a group-IV homojunction systematicallyinduced an artificial valence-band discontinuity when the anion wasdeposited first. It was also reported that in the case of Si—Sihomojunctions with Al—P or Ga—P intralayers, a reversal of the interfacedeposition sequence led to a reversal of the valence-band discontinuity,as expected.

In the second article it was shown, again using x-ray photoemission,that a similar band offset effect can be induced using Al—As as a“dipolar intralayer” between two regions of {111}-oriented germanium.Specifically, an offset of 0.4 eV was obtained for the “anion-first”Ge(substrate)-As—Al—Ge(overlayer) sequence, consistent with the“anion-first” As—Ga sequence reported by McKinley, the overlayercomponent exhibiting a lower binding energy with respect to thesubstrate component. In the third article, multiple III-V bilayer(intralayer) stacks were investigated. The measured value of valenceband offset remained the same, 0.5 eV, for an individual double layer,for double-stacked bilayers and for triple-stacked bilayers. Experimentsperformed on 2(Ga—P) and 2(P—Ga) were fully consistent with those on2(Al—P) and 2(P—Al); no substantial increase was observed on going fromthe individual bilayers to two bilayers or even to three bilayers. Itwas therefore concluded that stacked interfacial III-V bilayers do notincrease the effect of an individual bilayer, contrary to elementarypredictions based on sequential dipoles.

In U.S. Pat. Nos. 7,084,423, 7,176,483, 7,462,860, and 7,884,003 and inpending U.S. patent application 2011/0169124, Grupp and Connellydescribed metal-semiconductor contacts having an interfacial layer atthe interface between a metal and a group IV semiconductor for thepurpose of reducing the Schottky barrier at the contact and, hence,reducing the specific resistivity of the contact. A monolayer of arsenic(or nitrogen) was included amongst the possibleembodiments/specifications of the interfacial layer.

SUMMARY OF THE INVENTION

It is a distinct feature of the present invention that deliberatelyintroduced group V or group III atoms (or group II or group VI atoms)are organized in single ordered (e.g., epitaxially oriented) interfacialmonolayers. Moreover the present invention provides a process andstructure wherein a metal contact is deposited and not necessarilyformed by silicidation, a feature that allows a much broader range ofmetals to be used for formation of metal-semiconductor contacts,particularly metals that have favorable properties over metal silicidesfor specific applications such as higher electrical conductance oroptical transparency or ferromagnetism. The highest possible metalconductivity is desired in metal source/drain field effect transistorsas these devices are scaled down in size to have critical dimensions(for example source width and height) of 20 nanometers or less.Efficient spin injection into semiconductors from ferromagnetic metalssuch as gadolinium is required for devices such as spin effecttransistors in so-called spin-electronics (“spintronics”) applications.A spin-metal-oxide-semiconductor field effect transistor (spin-MOSFET)with ferromagnetic metal source and drain and a group IV semiconductorchannel is an example of a spin effect transistor. In emissive displaysit is often desirable to have a metal contact that allows goodtransmission of emitted light (high transparency) yet at the same timeforms a low resistance contact to the active material. Conversely inphotonic devices such as semiconductor lasers or modulators, it may bedesirable to have metal contacts that are not transparent so as tominimize losses due to light absorption. Metal silicides have theundesirable property of being somewhat transparent, with the consequencethat optical energy may enter a silicide region located within theoptical field of a photonic element and subsequently be absorbed in thesilicide.

The present invention does not require doping of the semiconductorproximate to the metal contact, although it may be practiced inconjunction with semiconductor doping. Nor does the present inventionrequire a metal silicidation step. Devices configured in accordance withembodiments of the present invention include at least an orderedmonolayer of a group V and/or an ordered monolayer of a group IIIelement or elements at the interface between a semiconductor and a metalcontact. The metal is deposited after the formation of at least anordered monolayer of interfacial atoms.

Embodiments of the present invention provide electrical contacts havingone or more monolayers disposed between a group IV semiconductor and ametal, the semiconductor characterized by a crystal lattice structureand a monolayer consisting of a single atomic layer of atoms of one ormore group V materials or a single atomic layer of atoms of one or moregroup III materials, each single atomic layer being in epitaxialalignment with one another and with the semiconductor lattice; andmethods of forming such electrical contacts.

Further embodiments of the invention provide an electrical contact thatincludes a metal and a group IV semiconductor separated by a monolayerof group V atoms and, optionally, a monolayer of group III atoms at aninterface between the metal and the semiconductor. The metal may be madeof atoms of the same metallic element as the monolayer of group IIImetal atoms or of atoms of a different metallic element than themonolayer of group III metal atoms. In some instances, the group IIIatoms may be any one or more of aluminum, gallium, indium or boron, ormixtures of aluminum, gallium, boron and/or indium. The group IVsemiconductor may be germanium, silicon, an alloy of germanium andsilicon or germanium and tin, or an alloy or compound of silicon and/orgermanium with carbon. The group V atoms may be any one or more ofnitrogen, phosphorus, arsenic or antimony. In some instances, onemonolayer of group III atoms will be immediately adjacent the surface ofthe group IV semiconductor. In other cases, one monolayer of group Vatoms will be immediately adjacent the surface of the group IVsemiconductor. The surface of the group IV semiconductor may be a{111}-oriented surface or a {100}-oriented surface.

The present invention also includes methods of forming electricalcontacts such as those described above. In some instances, this involvesa {100}-oriented surface of the group IV semiconductor being etched witha crystallographically selective etch to reveal and expose one or more{111}-oriented semiconductor crystal facets; the monolayer of group Vatoms being formed on the {111} facets; and the monolayer of group IIIatoms being subsequently deposited on the monolayer of group V atoms.The monolayers of group V atoms and/or group III atoms may be producedby way of separate vapor deposition processes or by separate chemicalreactions. For example, in a process conducted under ultra-high vacuum(UHV) conditions, prior to depositing the group V atoms or the group IIIatoms, as appropriate, the {111}-oriented facets of the semiconductormay be cleaned in situ and the semiconductor heated to a sufficientlyhigh temperature to obtain a 7×7 reconstruction in the case of a {111}silicon surface or a 5×5 reconstruction in the case of a {111} silicongermanium surface or a 2×8 reconstruction of a {111} germanium surfaceafter which the semiconductor may be heated to an elevated temperatureduring deposition of the group V atoms and/or group III atoms. Afterforming a first monolayer of group V atoms and a first monolayer ofgroup III metal atoms, metal atoms may be deposited on the firstbi-layer (two monolayers) directly or further monolayers of group Vatoms and/or group III atoms may be added to create a stack ofmonolayers in excess of a single bi-layer before metal atoms aredeposited to form a contact.

These and further embodiments of the present invention are described ingreater detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and notlimitation, in the figures of the accompanying drawings, in which:

FIGS. 1(a) and 1(b) illustrate potential barriers at metal-semiconductorjunctions; specifically FIG. 1 (a) shows a semiconductor (to theleft)—metal (to the right) interface with a fixed and thick barrier toelectron flow; and FIG. 1(b) illustrates how a dipole layer interposedbetween the metal and semiconductor has eliminated the barrier exceptbetween one pair of atomic planes.

FIG. 2 illustrates an example of a process for forming a very lowresistance metal contact to a semiconductor surface in accordance withan embodiment of the present invention.

FIGS. 3(a), 3(b) and 3(c) provide views of a 7×7 reconstructed{111}-oriented silicon surface.

FIG. 4 illustrates an example of group V atoms bonded directly withexposed silicon surface atoms to form a fully coordinated latticetermination with no dangling bonds.

FIG. 5 illustrates a bi-layer (two monolayers) interposed on a (111)surface of an n-type semiconductor, producing a contact by the processillustrated in FIG. 2 in accordance with embodiments of the presentinvention.

FIGS. 6(a) and 6(b) illustrate double bi-layers on (111) interfaces forn-type semiconductors with fields across long or short interplanarseparations, respectively.

FIG. 7 illustrates double bi-layers, as in FIG. 6, but for p-typesemiconductors, providing an extremely low resistance to the electricalconduction of holes through the contact, in accordance with furtherembodiments of the present invention.

FIG. 8 illustrates a process for creating the contact shown in FIG. 7 inaccordance with embodiments of the present invention.

FIG. 9 illustrates one bi-layer (two monolayers), as in FIG. 5, but fora {100} semiconductor surface, rather than a {111} surface.

FIGS. 10 and 11 illustrate experimental Schottky diode current—voltagecharacteristics obtained from aluminum—{111} oriented p-type siliconcontacts and compares measured data from contacts with an arsenic atomicmonolayer at the interface with data from contacts with no arsenicinterfacial layer.

DETAILED DESCRIPTION

In light of the challenges described above, the present inventors haverecognized a need for a metal contact technology that can reduce theresistance of metal contacts to doped S/D regions or, alternatively, ametal-semiconductor technology that eliminates, as much as possible, theSchottky barrier between the metal and the semiconductor. A lowresistance metal-semiconductor contacting technology will haveapplication wherever low resistance is required, for example in solarcell applications and in metal S/D field effect transistors (FETs). Thepresent invention relates to techniques for reducing the specificcontact resistance of metal-semiconductor (group IV) junctions byinterposing a monolayer of group V or group III atoms at the interfacebetween the metal and the semiconductor, or interposing a bi-layer madeof one monolayer of group V atoms and one monolayer of group III atoms,or interposing multiple such bi-layers. The invention includes methodsfor forming such a metal-semiconductor contact of very low barrierheight (approaching zero) and extremely low specific contact resistanceby providing at least a single ordered layer of atoms at the interfacebetween a metal and a semiconductor. The resulting low specificresistance metal—group IV semiconductor junction finds application as alow resistance electrode in semiconductor devices including electronicdevices (e.g., transistors, diodes, etc.) and optoelectronic devices(e.g., lasers, solar cells, photodetectors, etc.) and/or as a metalsource and/or drain region (or a portion thereof) in a FET. Themonolayer of group V or group III atoms adjacent to the semiconductorsurface is predominantly an ordered layer of atoms formed on the surfaceof the group IV semiconductor and chemically bonded to the surface atomsof the group IV semiconductor.

The present invention's emphasis on ordered monolayers, as well as theinclusion of group V elements, such as phosphorus or antimony, and groupIII elements, such as aluminum, boron, gallium or indium, distinguishesit from the earlier work of Grupp and Connelly (cited above). Further,the above-cited works of Marsi et al. and McKinley et al. specified anintention to create an energy band offset between two regions ofsemiconductor and no mention was made of modifying a Schottky barrierbetween a metal and a semiconductor or even of the possibility of doingso.

As described below, where both the group III and group V atoms arepresent, the resulting bi-layer provides an electrical dipole betweenthe semiconductor and the bulk metal. A similar dipole exists when onlya single layer of group V atoms is present, as an image charge is formedin the bulk metal. Further, in some instances, multiple bi-layers may beused between the semiconductor and the bulk metal (e.g., 2 or 3 suchbi-layers). Indeed, dipole layers can be added until the extra energyfrom increasing the field causes the atoms to rearrange themselves.

Further, although monolayers of pure group V or group III substances aredescribed herein, some embodiments of the present invention may make useof monolayers that comprise atoms of more than one type of group V atoms(for example a mixture of arsenic and phosphorus atoms within amonolayer) or more than one species of group III atoms. Hence,references to monolayers (whether part of a bi-layer or otherwise) bothbelow and in the claims should be read as encompassing monolayers of asingle kind of group V or group III atoms and as monolayers of group Vor group III atoms of more than one element.

In the examples described herein, the semiconductor is a group IVsemiconductor, for example germanium, silicon, an alloy of silicon andgermanium, or an alloy comprising two or more of the elements silicon,germanium, carbon and tin. FETs or other electronic devices made up ofcompound semiconductors may also benefit from use of low resistancejunctions provided in accordance with the present invention. Also in thefollowing examples, the metal forming the junction with thesemiconductor (and the interfacial layer of ordered group V atoms) isdescribed as a group III metal. However, this need not necessarily bethe case. It is not necessary that the metal is a group III metal. Othermetals, e.g., low work function metals such as magnesium, lanthanum,ytterbium or gadolinium, may also be used for obtaining low electronpotential (energy) barriers or high hole potential barriers between themetal and the semiconductor. Alternatively high work function metalssuch as nickel, platinum, iridium or ruthenium may be preferred forobtaining low hole barriers or high electron barriers between the metaland the semiconductor. However this does not preclude the use of higherwork function metals such as platinum or ruthenium for making contactswith low electron barriers either. The energy barrier between the metalFermi level and the semiconductor conduction band can be low despite themetal having a high work function by virtue of the large magnitude ofthe dipole created by the presence of the ordered group V monolayer atthe semiconductor interface.

It may be advantageous in many applications to use the same metal formaking contact to both p-type and n-type doped semiconductor regions,for example in forming source and drain contacts in both p-channel FETsand n-channel FETs. Moreover, it may be very advantageous for the metalto be a barrier metal such as tantalum nitride (TaN) or titanium nitride(TiN) or ruthenium (Ru) and for the same barrier metal to be used inmaking contact to both p-type and n-type semiconductor regions. In suchcases where the same metal is used to form low barrier contacts to bothn-type and p-type semiconductor, the interfacial monolayer chemicallybonded to the semiconductor surface will be an interfacial layer ofordered group V atoms at n-type contacts and will be an interfaciallayer of ordered group III atoms at p-type contacts. Similarly, wherethe same metal is used to form metal sources and/or drains of bothn-channel and p-channel metal source/drain MOSFETs, the interfacialmonolayer chemically bonded to the semiconductor surface will be aninterfacial layer of ordered group V atoms at source/drain junctions ofn-channel MOSFETs and will be an interfacial layer of ordered group IIIatoms at source/drain junctions of p-channel MOSFETs.

Ferromagnetic metals such as gadolinium, iron, nickel or cobalt oralloys of these elements or ferromagnetic alloys of manganese may beused to obtain metal-semiconductor contacts with high spin injectionefficiency. In specific applications where high electron spin injectionefficiency is desired, the interfacial monolayer chemically bonded tothe semiconductor surface is preferably an interfacial layer of orderedgroup V atoms. The ferromagnetic metal may be deposited directly on tothe group V monolayer or a monatomic layer of group III metal atoms maybe chemically bonded to the group V atoms and the ferromagnetic metaldeposited on to the group III monolayer.

Also other metallic materials including alloys of pure metals, metalsilicides such as nickel silicide of composition Ni₂Si, NiSi or NiSi₂ orplatinum silicide or cobalt silicide, or even semi-metals may be usedwherein the metallic material is directly adjacent to the group V orgroup III monolayer. It is possible and may be most convenient inmanufacturing for the same metallic material to be applied to bothn-type and p-type semiconductor contacts or as the metal source and/ordrain of both n-channel and p-channel MOSFETs.

To obtain the desired metal-semiconductor contact with extremely lowbarrier height to electrons and an extremely low resistance to theelectrical conduction of electrons through the contact, the singleordered layer of atoms is a single ordered layer of group V atoms. Thegroup V atoms may be nitrogen atoms, phosphorus atoms, arsenic atoms orantimony atoms or a mixture of these group V atoms. In one embodiment ofthe invention, the monolayer of group V atoms is a layer of arsenicatoms ordered in epitaxial (or substantially epitaxial) alignment withthe germanium or silicon or group IV semiconductor alloy crystallattice. Such a contact with extremely low resistance to conduction ofelectrons is used for making electrical contact to n-type dopedsemiconductor such as the n-type doped source and drain regions of ann-channel FET or for making metal source/drain regions that make directcontact to an electron channel in an n-channel FET.

In many cases, the surface of the group IV semiconductor on which themetal contact is formed will be a {111}-oriented surface and, to thegreatest possible extent, each of the group V atoms in the singleordered layer of atoms is chemically bonded in a three-way coordinationwith atoms in the {111} oriented surface of the semiconductor. In otherinstances, however, the contacted surface of the group IV semiconductorwill be a {100} or {110} surface. In some instances a {100} surface maybe preferred.

Before discussing embodiments of the present invention in detail, it ishelpful to review some of the underlying theory. At interfaces ofcontacts between metals and semiconductors, the Fermi energy in themetal is observed to be “pinned” at a specific energy in thesemiconductor energy band gap for each semiconductor causing a barrierbetween the metal Fermi level and the conduction band or valence band inthe semiconductor. Though the semiconductor can be made conducting(e.g., with doping), fixing the Fermi energy E_(F) near thesemiconductor band edge E_(c) in the bulk crystal (E_(F), without avoltage applied, is uniform through the system), as illustrated in FIG.1(a), E_(c) remains well above E_(F) at the interface. As a consequence,the region of the semiconductor near the interface has not been made agood conductor. Current is carried only weakly between the metal and thestrongly conducting region of the semiconductor. The conduction ofelectron current would be by thermionic emission into the conductionband (excitation over the barrier), or by tunneling through the barrier,which will often be even smaller as the barrier may be many tens ofAngstroms wide. More generally, current may be conducted between themetal and semiconductor by so-called “thermionic field emission” whichis a combination of thermionic emission and tunneling of electronsthrough the energy barrier.

The present invention seeks to eliminate, or at least sharply reduce,this barrier by inserting an electric dipole layer between the metal andthe semiconductor, shifting the relative positions of the band edge andthe Fermi energy at the interface. The resulting energies areillustrated in FIG. 1(b). The net result is to remove almost all of thebarrier region, except for that remaining in the dipole layer.

How this is accomplished for a silicon-metal interface is most simplyunderstood in terms of “theoretical alchemy”, as described in W. A.Harrison, Elementary Electronic Structure, World Scientific (Singapore,1999), revised edition (2004) and in the article Polar HeterojunctionInterfaces, by W. A. Harrison et al., Phys. Rev. B 18, 4402 (1978). Oneimagines removing a proton from the nucleus of each silicon atom in thelast plane before the metal, converting it to an aluminum nucleus (oneelement to the left in the Periodic Table) and inserting that proton inthe nucleus in the next to last plane of the silicon lattice, convertingit to a phosphorous nucleus. This effectively produces a sheet ofnegative charge in the last plane of atoms before the metal and a sheetof positive charge in the next to last plane and results in a dipolewith a large electric field between the two planes of atoms. This fieldactually polarizes the bonds in this layer, reducing it by a factor ofthe reciprocal of the dielectric constant ( 1/12=0.083 for silicon), butstill a large field and a large shift in electrical potential results asillustrated in FIG. 5(a). In fact, not only are the bonds within thedipole layer polarized, so too are the bonds in neighboring layers,modifying the effective charges of all atoms in the region, modifyingthe fields as shown in FIG. 5(b). It however leaves a very similar netshift in electric potential (estimated to be 1.39 eV in the case of(100) planes in silicon, with bond length d=2.35 Å), more than enough toremove the bulk barrier.

We could repeat the theoretical-alchemy process, removing another protonfrom the Al nucleus, making it a magnesium nucleus, and insert it in thephosphorus nucleus to make it a sulfur nucleus. The same concept appliesand this doubles the charge on each plane and doubles the dipole shift.It corresponds to inserting a plane of atoms from column II and a planeof atoms from column VI, rather than III and V. One could even apply ita third time, inserting a NaCl layer, but probably such a depositionwould not continue the silicon structure epitaxially as generally in ourinvention, but would very likely form a neutral NaCl rock-salt plane,without a dipole layer. On the other hand some noble-metal halides doform in the tetrahedral structure of silicon and these would be expectedto grow epitaxially, corresponding to a monolayer of a column VIIelement and a monolayer of a column IB (noble-metal) element and theestimated dipole shift would be three times that of the Al—P bi-layer.Thus our invention includes also dipole shifts from epitaxial layersfrom columns VI, VII, II, and IB, as well as columns V and III.

The result is not changed if, instead of theoretically converting thelast two planes of silicon atoms to phosphorous and aluminum, we insteadinterpose between the silicon and the metal actual single atomic layersof phosphorous, or any other column V element, and aluminum, or anothercolumn III element. Any suitable corresponding bi-layers of groupV-group III materials may be used for the purpose of eliminating (or atleast significantly reducing) the Schottky barrier and can be chosen forconvenience or other considerations, and similarly any element of thecolumns IB, II, VI, and VII serve as well as the ones mentioned in thepreceding paragraph. More specifically, an ordered monolayer of group VIelements sulfur and/or selenium and/or tellurium may be deposited incombination with an ordered monolayer of group II elements zinc and/orcadmium to form an ordered II-VI bi-layer.

Turning now to FIG. 2, one example of a process 10 for forming a verylow resistance metal contact to a semiconductor surface is illustrated.In this process, a {100}-oriented surface of the group IV semiconductor(or alloy or compounds of group IV semiconductors and/or carbon), 12, isetched with a crystallographically selective etch to reveal and exposeone or multiple {111}-oriented semiconductor crystal facets 14. Then, amonolayer of group V atoms is formed 16 on the {111} facets, followed bydeposition 18 of a suitable group III metal to form the contact. Note,the monolayer of group V atoms need not necessarily be a perfect orderedmonolayer. That is, the monolayer of group V atoms may have some gaps incoverage or some excess atoms. Stated differently, there may remain,after deposition of the ordered monolayer, some number of unsatisfieddangling bonds of the group IV semiconductor or a number of group Vatoms in excess of the number of previous dangling bonds of the group IVsemiconductor or some fraction of semiconductor or group V atoms at thesurface that are disordered and not in alignment with the semiconductorcrystal lattice. Nevertheless, in either instance this will still beconsidered an ordered monolayer of group V atoms for purposes of thepresent invention.

In an alternative process to the one described in FIG. 2, the metalatoms in step 18 may be metal atoms other than group III metal atoms.For example, the metal may be an alloy of pure metals, a metal silicideor a metallic compound.

The monolayer of group V atoms may be produced by way of a vapordeposition process or by a chemical reaction. In the case of a vapordeposition process, this may include exposing the semiconductor at anelevated temperature to a vapor flux of group V atoms or a flux ofmolecules of the group V element. The flux of group V atoms/moleculesmay be generated by thermally evaporating a source of the group Velement. In one embodiment of the invention, the flux is a flux ofarsenic molecules of composition As₄ and the As₄ molecular flux iscreated by the thermal evaporation of an elemental arsenic source in aKnudsen cell (k-cell) as is known in the practice of molecular beamepitaxy.

The various manufacturing tools that may be used for deposition of thegroup V and/or group III monolayers include molecular beam epitaxy(MBE), gas source molecular beam epitaxy (GSMBE), metalorganic molecularbeam epitaxy (MOMBE), metalorganic chemical vapor deposition (MOCVD),metalorganic vapor phase epitaxy (MOVPE), atomic layer deposition (ALD),atomic layer epitaxy (ALE) and chemical vapor deposition (CVD) tools,including plasma-enhanced CVD (PECVD) or photon or laser-induced CVD.

Another vapor deposition process that may be used in accordance withembodiments of the present invention involves the group V element atomsbeing deposited on the semiconductor surface by decomposition of a vaporphase compound of the group V element, for example a hydride of thegroup V element. Suitable group V hydride gases include ammonia, NH₃,for nitrogen atom deposition; phosphine, PH₃, for phosphorus; arsine,AsH₃, for arsenic and stibine, SbH₃, for the deposition of an antimonyatomic layer. Alternatively the vapor phase compound of the desiredgroup V element could be a metalorganic compound, examples of such beingan alkyl arsine such as tertiary butyl arsine for the deposition of anarsenic monolayer or an alkyl stibine such as triethylantimony(triethylstibine) for the deposition of an antimony monolayer.

In the case of a process carried out under ultra-high vacuum conditions,prior to being exposed to the group V atom or compound vapor flux, thesilicon with the {111}-oriented surface may be cleaned in situ andheated to a sufficiently high temperature to obtain a 7×7 reconstructionof the {111} silicon surface. FIGS. 3(a) (perspective view), 3(b) (planview of primitive unit cell) and 3(c) (side view of primitive unit cell)provide views of such a 7×7 surface 20. Atoms 22 represent atoms in theunderlying (1×1) bulk silicon material. Atoms 24 represent so-calledrest atoms (atoms one layer below the adatoms). Atoms 26 representdimers (paired surface silicon atoms). Atoms 28 represent the adatoms(silicon atoms laying on the crystal surface). Corner holes in thestructure are shown at 30.

Then, the silicon is maintained at a temperature in the range ofapproximately 20° C. to 750° C. (inclusive) during exposure to the groupV atom vapor or the group V compound molecular vapor. The siliconsurface may be exposed to the group V atom or compound molecular vaporflux for less than one second or several seconds or even severalminutes. With the silicon held at a suitable temperature, an orderedmonolayer of the group V atom is formed and, after so forming, themonolayer resists the deposition of additional group V atoms ordeposition of other atoms such as hydrogen or oxygen or carbon atoms.Alternatively the semiconductor temperature may be varied duringexposure to the group V atom vapor or the group V molecular compoundvapor, starting at a high temperature in the range 600 C to 800° C. andreducing to a lower temperature in the range 500 C to 20° C.

The group V atoms 32 (e.g., As, Sb or P) bond directly with the exposedsilicon surface atoms 34 to form a fully coordinated lattice terminationwith, to the greatest extent possible, no dangling bonds, as shown inFIG. 4, which is a side view of the resulting structure. Three of thefive valence electrons in each group V atom form bonds with siliconatoms at the surface of the group IV semiconductor and the remaining twovalence electrons form a “lone-pair” orbital as shown in theillustration.

Similar processes may be applied to obtain monolayers of group V atomson silicon surfaces other than the {111} orientation, such as a {100}orientated silicon surface. Similar processes may also be applied toobtain monolayers of group V atoms on group IV semiconductor surfacesother than silicon, such semiconductors including germanium, silicongermanium, silicon carbon, germanium tin or silicon germanium carbon.Moreover similar processes may also be applied to obtain monolayers ofgroup VI atoms on group IV semiconductor surfaces.

The exposure of the surface of a heated semiconductor to the group Vatom flux or compound molecular flux may be done in an ultra-high vacuum(UHV) chamber, in a vacuum chamber or in a reduced pressure chamber. Ifthe chamber in which the process occurs is not a UHV chamber, abackground or carrier gas may be present during the exposure. In oneembodiment, arsine, AsH₃ is delivered in dilute form in a gas mixtureconsisting primarily of hydrogen (H₂) or nitrogen (N₂). In semiconductormanufacturing, arsine is typically diluted at a concentration of a fewpercent or even as low as 100 parts per million or so in ultra purehydrogen or nitrogen. The arsine, whether it be pure arsine or a dilutemixture of one or a few percent arsine in hydrogen or nitrogen,decomposes at the heated semiconductor surface, liberating free arsenicatoms that bond directly with the exposed silicon surface to form afully coordinated lattice termination with no, or at least very few,dangling bonds.

A preferred process for the deposition of a monolayer of arsenic onsilicon from a hydride precursor gas (AsH₃) starts by heating thesilicon surface in a hydrogen ambient to a temperature sufficient toreduce any surface oxide then continues by heating the silicon surfaceto a temperature in the range 650° C. to 750° C. (most preferablybetween 675° C. and 725° C.) while exposing the surface to AsH₃ vaporfor a period of between 10 seconds to 30 minutes (most preferablybetween 20 seconds and 2 minutes). Such a process may be carried out ina CVD system or an ALD system and an ordered monolayer of arsenic atomsis formed. After so forming, the monolayer resists the deposition ofadditional group V atoms or deposition of other atoms such as hydrogenor oxygen or carbon atoms. Alternatively the semiconductor temperaturemay be varied during exposure to the AsH₃ vapor, starting at a hightemperature in the range 650° C. to 750° C. and reducing to a lowertemperature in the range 500° C. to 20° C.

As indicated above, it is not strictly necessary that the group V atomsform a perfect monolayer. A metal could be deposited on top of thisgroup V monolayer, or more silicon and then the metal. Thus themonolayer of charge may exist at the interfacial layer (as describedabove) or at the second, third or fourth planes from thesemiconductor-metal interface if one, two or three atomic layers ofsilicon respectively are deposited after the group V monolayer andbefore the metal. An advantage of having one or several atomic layers ofsilicon atoms between and therefore separating the monolayer of chargedgroup V atoms (ions) and the metal atoms is the increased magnitude ofthe charge dipole so created between the layers and hence the greaterreduction of the Schottky barrier for electrons at themetal-semiconductor junction. On the other hand, a disadvantage ofhaving one or several atomic layers of silicon atoms separating themonolayer of charged group V atoms (ions) and the metal atoms is thelarger spatial extent of the dipole region which is deleterious tocharge conduction through the barrier. For applications where a largeSchottky barrier to a p-type semiconductor is desired only advantagesare anticipated to result from including silicon atom layers between thegroup V atoms and metal atoms.

After forming the monolayer of coordinated group V atoms 38 on thesurface of a {111}-oriented group IV semiconductor 36, one monolayer ofa group III metal atoms 40 is deposited in the embodiment illustrated inFIG. 5, followed by the deposition of the metal contact (bulk metalatoms 42), providing the low barrier, low resistance metal contact, Inthis embodiment of the invention, the one layer of metal atoms 40 is alayer of group III metal atoms that may include aluminum, gallium orindium or a mixture of these group III metal atoms. In other embodimentsof the invention, metals or alloys of metals other than or incombination with a group III metal may be used. This monolayer of groupIII metal atoms is optional and need not necessarily be present in alljunctions formed in accordance with the present invention (a balancingnegative charge (described further below) would be an image chargeformed in the bulk metal).

Where present, the metal atoms in the one layer of metal atoms arepreferably coordinated with the monolayer of group V atoms alreadypresent on the semiconductor surface so as to form an ordered layer ofmetal atoms. Embodiments are possible, however, wherein the first layerof metal atoms is not strongly coordinated by chemical bonding to theunderlying ordered layer of group V atoms. The process then continues todeposit further metal atoms 42, the further atoms being of the samemetallic element as the first layer of metal atoms or atoms of a metalelement different from the first layer of metal atoms. FIG. 5 wouldillustrate the resulting structure if one makes atoms 40 and 42 the sameelement.

In FIG. 5, a single bi-layer that includes a monolayer of group V atoms38 and a monolayer of group III atoms 40 disposed between thesemiconductor atoms 36 and bulk metal 42 is represented. The two plots,(a) and (b), in the illustration represent the potentials at variouslocations across the junction, with plot (a) showing the first step inthe theoretical alchemy without the polarization of neighboring bondsand plot (b) taking such relaxation into account. Plot (b) is somewhatexaggerated to highlight the nature of the potentials experienced acrossthe junction.

The monolayer of group III metal atoms may be produced by a vapordeposition process or by a chemical reaction. For example, in the caseof a vapor deposition process, the one layer of metal atoms may beformed on the semiconductor surface by exposure of the surface to anatomic vapor flux of the group III metal element or to a vapor flux of acompound of the metallic element. Exposure may be for a duration of lessthan one second or a duration of several seconds or even severalminutes.

The vapor deposition process may involve exposure of the semiconductorwith the monolayer of Group V atoms to a vapor flux of metal atoms or aflux of molecules of the metallic element. The flux of metalatoms/molecules may be generated by thermally evaporating a source ofthe metal. In one embodiment of the invention, the flux is a flux ofaluminum atoms created by the thermal evaporation of an elementalaluminum source in a Knudsen cell (k-cell) as is known in the practiceof molecular beam epitaxy or evaporation of an elemental aluminum sourceby heating with an electron beam. The semiconductor may be heated duringthe deposition of the metal atoms. In an alternative vapor depositionprocess, the metal atoms may be deposited on the semiconductor surfaceby decomposition of a vapor phase chemical compound of the metal, forexample a metalorganic compound. Such a process may be classified mostgenerally as a chemical vapor deposition process. Suitable metalorganiccompounds of aluminum include trimethyl aluminum. Deposition of amonolayer of metal atoms from decomposition of a chemical vapor sourceis known more specifically as atomic layer epitaxy if the metal atomsenter into an epitaxial alignment to the semiconductor crystal latticeor atomic layer deposition if the metal atoms do not. In anotheralternative vapor deposition process, the metal atoms may possibly bedeposited by sputtering of the metal atoms from a solid source in whatis known as a physical vapor deposition (PVD) process.

After depositing the one layer of metal atoms, processing may continueby depositing additional layers of metal atoms (which may be the samemetal as the monolayer of group III atoms or a different metal). Thefurther additional layers of metal atoms may be of elemental compositionand thickness in accordance with the requirements of the specificapplication of the resulting metal-semiconductor contact. For examplefor contacts to nanometer-scale FETs, the additional layers of metalatoms may be layers of a barrier metal such as tantalum nitride,titanium nitride or ruthenium. In this context and in the commonterminology of the microelectronics industry, a barrier metal is a thinmetal layer usually deposited by a conformal deposition technique suchas atomic layer deposition (ALD), plasma enhanced ALD or chemical vapordeposition (CVD) that provides a barrier to the diffusion of a coppermetallization layer into the semiconductor. Alternatively the barriermetal may be deposited in an electrochemical deposition process or byreactive physical vapor deposition (PVD) wherein the metal is sputteredfrom a solid source or target. In alternative embodiments the additionallayers of metal atoms may constitute a metal silicide such as a nickelsilicide of composition Ni2Si, NiSi or NiSi2 or a platinum silicide ornickel-platinum silicide or cobalt silicide wherein the metal silicideis directly adjacent to the group V monolayer or to the group V—groupIII bi-layer.

In addition to depositing a monolayer of a group V material such asarsenic, phosphorus, etc., on a silicon surface, as previouslydiscussed, it may be advantageous to deposit some of the group Vmaterial at a sufficiently high temperature that some of the atoms enterthe silicon itself. Alternately, the silicon surface can be prepared inother known ways so that group V atoms are present near the siliconsurface. After this, the group V material is deposited in an appropriatemanner for the monolayer to form on the silicon surface. The objectiveof this is that the additional group V atoms in the siliconadvantageously form additional dipoles with image charges in the metaldeposited on the monolayer of group V material, favorably increasing theoverall dipole effect.

FIGS. 6(a) and 6(b) are further examples of metal-semiconductor contactsconfigured in accordance with embodiments of the present invention. InFIG. 6(a), contact 44 is similar to the one illustrated in FIG. 5, butincludes an additional bi-layer of a group V element and a group IIImetal. The electrical dipole is created across the long interlayerseparation (i.e., the relatively long distance between the constituentmonolayers 38 and 40 of the bi-layer). In FIG. 6(b), contact 44′ has anelectrical dipole across the short interlayer separation (i.e., therelatively short distance between the constituent monolayers 38 and 40of the bi-layer).

As shown in FIG. 7, to obtain a metal-semiconductor contact withextremely low barrier height to holes and an extremely low resistance tothe electrical conduction of holes through the contact, the singleordered layer of atoms is a single ordered layer of metal atoms 40 andincludes a single atomic layer of group V atoms 38 chemically bonded tothe monolayer of metal atoms and separated from the semiconductor 36surface atoms by the monolayer of metal atoms 40. In some embodiments,the single atomic layer of metal atoms is a monolayer of group III metalatoms that may be aluminum atoms, gallium atoms or indium atoms or amixture of these group III metal atoms. In some cases the monolayer ofgroup III metal atoms is a layer of indium atoms ordered in epitaxial(or substantially epitaxial) alignment with the germanium or silicon orgroup IV semiconductor alloy crystal lattice and an adjacent monolayerof group V atoms is chemically bound to the monolayer of metal atoms.The group V atoms may be nitrogen atoms, phosphorus atoms, arsenic atomsor antimony atoms or a mixture of these group V atoms. In some cases themonolayer of group V atoms is a layer of arsenic atoms ordered inalignment with and chemically bonded to the group III metal atoms thatform a single atomic layer in crystallographic alignment with andchemically bonded to the surface atoms of the germanium or silicon orgroup IV semiconductor alloy crystal lattice. In the illustration, twobi-layers between the semiconductor and the bulk metal are shown, butembodiments that include a single bi-layer are also contemplated withinthe scope of the present invention.

In some embodiments where it is required to form an extremely lowresistance contact to a p-type semiconductor or to provide an extremelyhigh conductance source and/or drain in a p-channel field effecttransistor, the contacted surface is a {111}-oriented semiconductorsurface. In other embodiments the contacted surface of the semiconductoris a {100}-oriented surface.

FIG. 8 illustrates a process 45 for creating the contact shown in FIG.7. Beginning with a {100}-oriented semiconductor surface 46, the (100)surface is etched with a crystallographically selective etch to revealand expose one or multiple {111} oriented semiconductor crystal facets48. A monolayer of group III metal atoms is formed on the {111} facets50, followed by deposition of a monolayer of group V atoms 52.Obviously, one can start the process directly from a {111} surface thatmay already be present as a result of an alternate device geometry orother considerations.

After depositing the monolayer of group V atoms, the process continuesby depositing further multiple layers of metal 54. The furtheradditional layers of metal atoms may be of elemental composition andthickness in accordance with the requirements of the specificapplication of the resulting metal-semiconductor contact, as previouslydescribed for the formation of contacts to n-type semiconductors withextremely low resistance to electron conduction.

The monolayer of group III metal atoms may be produced by a vapordeposition process or by a chemical reaction. In the case of a vapordeposition process, the semiconductor is exposed to a vapor flux ofgroup III metal atoms or a flux of molecules of a compound of the groupIII metal element. The flux of group III atoms/molecules may begenerated by thermally evaporating a source of the group III element. Inone embodiment of the invention, the flux is a flux of indium atomscreated by the thermal evaporation of an elemental indium source in aKnudsen cell (k-cell) as is known in the practice of molecular beamepitaxy. In an alternative vapor deposition process, the group IIIelement atoms are deposited on the semiconductor surface bydecomposition of a vapor phase compound of the group III element, forexample a metalorganic compound of the group III element. Decompositionof a vapor phase precursor compound of the group III metal may beachieved by heating of the semiconductor surface. Where it is preferrednot to heat the semiconductor surface to very high temperatures,decomposition may be achieved by plasma in a plasma-enhanced CVD (PECVD)or plasma-enhanced ALD (PEALD) type of tool and process. Alternativelydecomposition of the metal precursor may be achieved by a photon-inducedprocess.

The semiconductor with the {111}-oriented surface may be cleaned in situbefore it is exposed to the group III atom or group III molecularcompound vapor flux, for example by heating it to a sufficiently hightemperature under ultra-high vacuum conditions to obtain for the case ofsilicon a 7×7 reconstruction of the {111} silicon surface. Then thesemiconductor is maintained at a temperature in the range 20° C. to 750°C. inclusive during exposure to the group III atom vapor or the groupIII molecular compound vapor. Alternatively the semiconductortemperature may be varied during exposure to the group III atom vapor orthe group III molecular compound vapor, starting at a high temperaturein the range 600 C to 800° C. and reducing to a lower temperature in therange 500 C to 20° C.

The semiconductor surface may be exposed to the group III atom orcompound vapor flux for less than one second or several seconds or evenseveral minutes. The group III atoms bond directly with the exposedgroup IV semiconductor surface to form a monolayer of group III atoms,the group III atoms being to the greatest extent possible incrystallographic alignment with the semiconductor lattice.

The exposure of the surface of the semiconductor to the group III atomflux or molecular compound vapor flux may be done in a UHV chamber, in avacuum chamber or in a reduced pressure chamber. If the chamber in whichthe process occurs is not a UHV chamber, a background or carrier gas maybe present during the exposure. In one embodiment, a metalorganiccompound precursor such as trimethyl indium is delivered in dilute formin a gas mixture consisting primarily of a carrier gas such as hydrogenor nitrogen and decomposes at the heated semiconductor surface,liberating indium atoms that bond directly with the exposed silicon. Inanother embodiment, the metalorganic compound is trimethyl aluminum ortrimethyl gallium, which reacts at the heated semiconductor surface toform a monolayer of aluminum or gallium atoms, respectively.

After forming the monolayer of coordinated group III metal atoms on thesurface of a {111}-oriented group IV semiconductor, formation of the lowbarrier, low resistance metal contact continues by deposition of onelayer of group V atoms. The group V atoms in the one layer of group Vatoms are preferably coordinated with the monolayer of group III metalatoms already present on the semiconductor surface so as to form anordered layer of group V atoms. The process then continues to depositfurther metal atoms, the further atoms being of the same metallicelement as the first layer of metal atoms or atoms of a metal elementdifferent from the first layer of metal atoms.

In another embodiment of the invention, after forming the monolayer ofcoordinated group III metal atoms on the surface of a {100} or{111}-oriented group IV semiconductor, formation of the low barrier, lowresistance metal contact continues by deposition of a metal over themonolayer. The metal is not necessarily a group V metal. The metal maybe one having desirable properties such as structural or chemicalstability to ensure reliability of the electrical contact or device soformed. Examples of stable metals for contacts include platinum (Pt),tungsten (W) and the previously described “barrier metals” TaN, TiN andRu. A metal could be deposited directly on top of the group IIImonolayer, such that the group III monolayer is exactly at the interfacebetween the metal and the semiconductor or alternatively the group IIImonolayer may be separated from the metal by one or two monolayers ofthe group IV semiconductor. Thus the monolayer of charge associated withthe group III monolayer may exist at the interfacial layer or at thesecond or third planes from the semiconductor-metal interface if one ortwo atomic layers of group IV semiconductor respectively areintermediate between the group III monolayer and the metal. An advantageof having one or several atomic layers of silicon atoms between andtherefore separating the monolayer of charged group III atoms (ions) andthe metal atoms is the increased magnitude of the charge dipole socreated between the layers and hence the greater reduction of theSchottky barrier at a metal −p-type semiconductor junction or at a metalto p-channel source/drain junction of a MOSFET.

Another embodiment of the invention forms a metal semiconductor contactwith a monolayer of group V or group III atoms at themetal-semiconductor interface where the group V (for example arsenic)monolayer or group III (for example boron) monolayer is formed bysegregation of group V or group III atoms out of a layer of material incontact with the semiconductor surface. The layer of material may bedeposited on the semiconductor surface, for example by CVD or PVD. Thegroup V atoms may be introduced into the layer of material by includingthem as a dopant in the CVD or PVD deposition process or by ionimplantation. Alternatively the layer of material may be formed byreaction of another element or elements with the semiconductor surfacein which case the group V or group III atoms may be implanted eitherbefore or after the material is formed by chemical reaction. For examplethe layer may be silicon oxide or silicon nitride formed by thermaloxidation of a silicon surface and the group V or group III atoms may beintroduced into the silicon oxide or nitride layer by ion implantation.In another embodiment the layer may be a deposited thin film of dopedsilicon oxide containing a high concentration of a group V element suchas phosphorus or a group III element such as boron. The former isgenerally known as a “phosphosilicate glass” (“PSG”) and the latter as a“borosilicate glass” (“BSG”) and methods for deposition of these dopedsilicate glasses (such as CVD) are well known and widely practiced inthe microelectronics industry. Alternatively the layer of material maybe a metal silicide formed by reaction of a metal with a silicon surfaceand the group V or group III atoms may be introduced into the metalsilicide layer by ion implantation.

After introducing a concentration of group V or group III atoms in thelayer of material in contact with the semiconductor surface, the wholelayer structure is annealed at a sufficiently high temperature to causethe group V or group III atoms to segregate to the interface form anordered monolayer of group V or group III atoms at the interface withthe group V or group III atoms bonded in epitaxial coordination with thetop layer of semiconductor atoms. In a case where the layer of materialis a doped silicon oxide (e.g. PSG or BSG) or silicon nitride and thesemiconductor is silicon, after the anneal cycle has caused segregationof some group V or group III element to the silicon—silicon oxide (ornitride) interface, the silicon oxide (or nitride) is subsequentlyremoved by selective wet chemical etching, leaving behind a coordinatedmonolayer of the group V or group III atoms at the semiconductorsurface, and a metal is deposited to form a metal contact to thesemiconductor. In a case where the layer of material is a metal silicideand the semiconductor is silicon, after the thermal cycle has caused theinterfacial segregation of the group V or group III element to form aninterfacial ordered monolayer, the metal silicide may be removed or maybe retained in place to function as the metal contact itself.

Still further embodiments of the present invention involve the use of a{100}-oriented semiconductor surface. FIG. 9 illustrates an example of acontact that includes such a surface, this contact including a monolayerof group V atoms that has been deposited on the group IV semiconductor{100} surface using any of the above-discussed techniques. A monolayerof group III metal atoms is then deposited on the group V atoms,followed by deposition of further metal layers. These further metalatoms may be of the same metallic element as the first layer of metalatoms or atoms of a metal element different from the first layer ofmetal atoms. The metal-semiconductor contact shown in FIG. 9 providesextremely low barrier height to electrons and an extremely lowresistance to the electrical conduction of electrons through thecontact. If the contact is intended to provide extremely low barrierheight to holes and an extremely low resistance to the electricalconduction of holes through the contact, the positions of the group Vatoms and group III atoms in the bi-layer would be reversed with respectto one another.

Experimental Schottky diodes have been fabricated to illustrate theeffect of an arsenic interfacial monolayer on an examplealuminum-silicon Schottky barrier. The example experiments do notrepresent typical process conditions nor do they necessarily representoptimal process conditions. The illustrative experiments were done on{111}-oriented silicon wafers, doped p-type with a boron concentrationof approximately 1×10¹⁷ atoms/cm³. A first set of experimental Schottkydiodes was fabricated under ultra-high vacuum conditions and a secondset under low pressure chemical vapor deposition conditions in ahydrogen atmosphere.

The first set of diodes was processed as follows: After heating thesilicon to a high temperature above 800° C. in ultra high vacuum toclean and reconstruct the {111} Si surface to 7×7, the temperature wasreduced from 800° C. to 700° C. and then the silicon surface was exposedto a flux of arsenic molecules of type As₂ for ten minutes before theAs₂ flux was terminated. Rutherford back scattering analysis confirmedthat an areal density of arsenic equal to 7.30×10¹⁴ atoms/cm² resultedfrom this exposure, a value that is close to the known areal density7.83×10¹⁴ atoms/cm² of surface atoms on a 1×1 reconstructed {111}silicon surface. As such it may be reasonably concluded thatapproximately a single monolayer of arsenic atoms had been deposited.After cooling to room temperature, a layer of pure aluminum wasdeposited in the same ultra high vacuum system and subsequentlypatterned to provide simple diode structures that could be measuredelectrically. For comparison purposes, a similar wafer was processedthrough a similar sequence of steps except without any deliberateexposure of the silicon surface to arsenic. FIG. 10 shows representativemeasured current versus voltage characteristics of these experimentaldiodes, taking equally sized diodes from each wafer (with and withoutthe arsenic). Diodes on wafers without arsenic at the interfaceconsistently exhibit a relatively small Schottky barrier height to thep-type silicon as indicated by the measured curve 72 in FIG. 10. Fromcurve 72, a barrier height may be extracted by fitting a standard diodeequation (thermionic emission model) to the measured data. The extractedbarrier height for diodes without arsenic exposure is 0.40 eV (with anexperimental error of approximately 0.03 eV), which is consistent withpublished values of barrier height for intimate aluminum contacts onp-type silicon. Diodes on wafers that had the silicon interface exposedto arsenic so as to form a monolayer consistently exhibit a largerSchottky barrier height to the p-type silicon as indicated by data curve70 in FIG. 10. The larger barrier height to p-type silicon is indicativeof a smaller barrier height to n-type silicon according to the generalrule that the sum of magnitudes of the n-type and p-type barrier heightsis very close to the silicon band gap. Thus it is demonstratedexperimentally that a monolayer of arsenic introduced at the interfacebetween aluminum and a {111} oriented silicon surface does provide alarger Schottky barrier to p-type silicon consistent with a reducedelectron barrier between the aluminum Fermi level and the conductionband of the silicon (i.e. consistent with a reduced Schottky barrierheight to n-type silicon).

The second set of diodes was processed as follows: After heating thesilicon to 900° C. in a flow of hydrogen gas to clean the {111} Sisurface, the temperature was reduced from 900° C. to 700° C. and thenthe silicon surface was exposed to a flux of arsine (AsH₃) molecules forten minutes with the temperature held at 700° C. before the AsH₃ flowwas terminated. The arsine was heavily diluted in hydrogen (H₂) at aconcentration of approximately 2 parts per million with a total gas flowof 20.4 liters per minute. Rutherford back scattering analysis confirmedthat an areal density of arsenic equal to 7.8×10¹⁴ atoms/cm² resultedfrom this exposure, a value that is close to the known areal density7.83×10¹⁴ atoms/cm² of surface atoms on a 1×1 reconstructed {111}silicon surface. As such it may be reasonably concluded thatapproximately a single monolayer of arsenic atoms had been deposited.After cooling to room temperature, a layer of pure aluminum wasdeposited by electron beam evaporation in a separate ultra high vacuumsystem and subsequently patterned to provide simple diode structuresthat could be measured electrically. For comparison purposes, a similarwafer was processed through a similar sequence of steps except withoutany deliberate exposure of the silicon surface to arsenic. FIG. 11 showsrepresentative measured current versus voltage characteristics of theseexperimental diodes (with and without the arsenic). Diodes on waferswithout arsenic at the interface consistently exhibit a relatively smallSchottky barrier height to the p-type silicon as indicated by themeasured curve 82 in FIG. 11. From curve 82, a barrier height may beextracted by fitting a standard diode equation (thermionic emissionmodel) to the measured data. The extracted barrier height for diodeswithout arsenic exposure is 0.42 eV (with an experimental error ofapproximately 0.03 eV), which is consistent with published values ofbarrier height for intimate aluminum contacts on p-type silicon. Diodeson wafers that had the silicon interface exposed to arsenic so as toform a monolayer consistently exhibit a larger Schottky barrier heightto the p-type silicon as indicated by data curve 80 in FIG. 11. Thelarger barrier height to p-type silicon is indicative of a smallerbarrier height to n-type silicon according to the general rule that thesum of magnitudes of the n-type and p-type barrier heights is very closeto the silicon band gap. Thus it is demonstrated experimentally that amonolayer of arsenic introduced at the interface between aluminum and a{111}-oriented silicon surface does provide a larger Schottky barrier top-type silicon consistent with a reduced electron barrier between thealuminum Fermi level and the conduction band of the silicon (i.e.consistent with a reduced Schottky barrier height to n-type silicon).

Thus, techniques for reducing the specific contact resistance of ametal—semiconductor junction by interposing a monolayer of group V orgroup III atoms, or multiple monolayers of group V and group III atomsat the interface between the metal and the semiconductor have beendescribed.

What is claimed is:
 1. A method of forming an electrical contact,comprising depositing, on a surface of a group IV semiconductor, whichhas been prepared so as to include atoms of a first group V materialnear the surface, a monolayer of a second group V material, the atoms ofthe second group V material being epitaxially aligned with a latticestructure of the semiconductor, and depositing a metal on the monolayerof the second group V material, there being formed between the atoms ofthe first group V material dipoles with image charges in the metal. 2.The method of claim 1, wherein the first group V material and the secondgroup V material are the same group V material.
 3. The method of claim1, wherein the group IV semiconductor is prepared by depositing thefirst group V material on the semiconductor at a sufficiently hightemperature that atoms of the first group V material enter thesemiconductor.
 4. The method of claim 1, wherein the monolayer of thesecond group V material is an imperfect monolayer, having gaps therein.5. The method of claim 1, wherein the monolayer of the second group Vmaterial is an imperfect monolayer, having excess second group Vmaterial atoms.
 6. The method of claim 1, wherein the monolayer of thesecond group V material is produced by way of a vapor deposition processthat includes exposing the group IV semiconductor to a vapor flux ofatoms of the second group V material or a flux of homonuclear moleculesof the second group V material generated by thermally evaporating asource of the second group V material, or by a chemical reaction.
 7. Themethod of claim 4, wherein the monolayer of the second group V materialis produced by way of a vapor deposition process that includes exposingthe group IV semiconductor to a vapor flux of atoms of the second groupV material or a flux of homonuclear molecules of the second group Vmaterial generated by thermally evaporating a source of the second groupV material, or by a chemical reaction.
 8. The method of claim 5, whereinthe monolayer of the second group V material is produced by way of avapor deposition process that includes exposing the group IVsemiconductor to a vapor flux of atoms of the second group V material ora flux of homonuclear molecules of the second group V material generatedby thermally evaporating a source of the second group V material, or bya chemical reaction.
 9. The method of claim 1, wherein atoms of thesecond group V material are deposited on the semiconductor surface bydecomposition of a vapor phase compound of the second group V material.10. The method of claim 9, wherein the second group V material is ahydride of one of ammonia, phosphine, arsine, or stibine.
 11. The methodof claim 4, wherein atoms of the second group V material are depositedon the semiconductor surface by decomposition of a vapor phase compoundof the second group V material.
 12. The method of claim 11, wherein thesecond group V material is a hydride of one of ammonia, phosphine,arsine, or stibine.
 13. The method of claim 5, wherein atoms of thesecond group V material are deposited on the semiconductor surface bydecomposition of a vapor phase compound of the second group V material.14. The method of claim 13, wherein the second group V material is ahydride of one of ammonia, phosphine, arsine, or stibine.
 15. The methodof claim 1, wherein the group IV semiconductor comprises any ofgermanium, silicon, an alloy of silicon and germanium, an alloy ofgermanium and tin, an alloy of silicon with carbon, a compound ofsilicon and carbon, an alloy of germanium with carbon, a compound ofgermanium and carbon.
 16. The method of claim 4, wherein the group IVsemiconductor comprises any of germanium, silicon, an alloy of siliconand germanium, an alloy of germanium and tin, an alloy of silicon withcarbon, a compound of silicon and carbon, an alloy of germanium withcarbon, a compound of germanium and carbon.
 17. The method of claim 5,wherein the group IV semiconductor comprises any of germanium, silicon,an alloy of silicon and germanium, an alloy of germanium and tin, analloy of silicon with carbon, a compound of silicon and carbon, an alloyof germanium with carbon, a compound of germanium and carbon.
 18. Themethod of claim 1, wherein the second group V material comprises any oneof nitrogen, phosphorus, arsenic and antimony; or a mixture of any twoor more of nitrogen, phosphorus, arsenic and antimony atoms.
 19. Themethod of claim 4, wherein the second group V material comprises any oneof nitrogen, phosphorus, arsenic and antimony; or a mixture of any twoor more of nitrogen, phosphorus, arsenic and antimony atoms.
 20. Themethod of claim 5, wherein the second group V material comprises any oneof nitrogen, phosphorus, arsenic and antimony; or a mixture of any twoor more of nitrogen, phosphorus, arsenic and antimony atoms.
 21. Themethod of claim 1, wherein the surface of the group IV semiconductor isa {111}-oriented surface or a {100}-oriented surface.
 22. The method ofclaim 4, wherein the surface of the group IV semiconductor is a{111}-oriented surface or a {100}-oriented surface.
 23. The method ofclaim 5, wherein the surface of the group IV semiconductor is a{111}-oriented surface or a {100}-oriented surface.
 24. The method ofclaim 1, wherein a {100}-oriented surface of the group IV semiconductoris etched with a crystallographically selective etch to reveal andexpose multiple {111}-oriented semiconductor crystal facets, and themonolayer of the second group V material is formed on the {111} facets.25. The method of claim 4 wherein a {100}-oriented surface of the groupIV semiconductor is etched with a crystallographically selective etch toreveal and expose multiple {111}-oriented semiconductor crystal facets,and the monolayer of the second group V material is formed on the {111}facets.
 26. The method of claim 5, wherein a {100}-oriented surface ofthe group IV semiconductor is etched with a crystallographicallyselective etch to reveal and expose multiple {111}-orientedsemiconductor crystal facets, and the monolayer of the second group Vmaterial is formed on the {111} facets.
 27. The method of claim 1,wherein the metal is one of: a metal silicide, a nickel silicide, acomposition NiSi, a platinum silicide, a barrier metal consisting oftantalum nitride or titanium nitride or ruthenium, or a ferromagneticmetal comprising one or more of nickel, cobalt, iron and/or gadolinium.28. The method of claim 4, wherein the metal is one of: a metalsilicide, a nickel silicide, a composition NiSi, a platinum silicide, abarrier metal consisting of tantalum nitride or titanium nitride orruthenium, or a ferromagnetic metal comprising one or more of nickel,cobalt, iron and/or gadolinium.
 29. The method of claim 5, wherein themetal is one of: a metal silicide, a nickel silicide, a compositionNiSi, a platinum silicide, a barrier metal consisting of tantalumnitride or titanium nitride or ruthenium, or a ferromagnetic metalcomprising one or more of nickel, cobalt, iron and/or gadolinium.